The term “high molecular weight polyethylene” is generally used to define polyethylene having a molecular weight of at least 3×105 g/mol as determined by ASTM 4020 and, as used herein is intended to include very-high molecular weight polyethylene or VHMWPE (generally characterized as polyethylene having a molecular weight of at least 1×106 g/mol and less 3×106 g/mol as determined by ASTM 4020) and ultra-high molecular weight polyethylene or UHMWPE (generally characterized as polyethylene having a molecular weight of at least 3×106 g/mol as determined by ASTM 4020).
High molecular weight polyethylenes are valuable engineering plastics, with a unique combination of abrasion resistance, surface lubricity, chemical resistance and impact strength. As a result, they find application in the production of high strength fibers for use in ropes and anti-ballistic shaped articles and in the production of other elongated articles, such as membranes for lithium batteries. However, since the flowability of these materials in the molten state decreases as the molecular weight increases, processing by conventional techniques, such as melt extrusion, is not always possible.
One alternative method for producing fibers and other elongated components high molecular weight polyethylene, including VHMWPE and UHMWPE, is by gel-spinning in which the polymer is dissolved in a solvent, the resultant gel is extruded into a fiber or membrane, and part or all of the solvent is then removed from the product. The dried product may be stretched at an elevated temperature below the melting point of the polyethylene to increase its tensile strength (i.e., tenacity) and tensile modulus (i.e., elastic modulus). Examples of such a process are disclosed in, for example, U.S. Pat. No. 5,741,451 and United States Patent Application Publication No. 2004/0161605.
Generally, the strength of polyethylene fibers increases with molecular weight but with conventional forms of polyethylene, such as those produced by Ziegler-Natta catalysts, the solubility of the material in conventional solvents, such as decalin and mineral oils, decreases as the molecular weight increases. In addition, increasing the molecular weight of the polyethylene can be accompanied by agglomeration during the gelation process. Thus the polymer particles tend to absorb the solvent causing the particles to swell and become entangled, thereby forming agglomerates. These more or less mobile networks of gel particles tend to stick to the surfaces of, for example, extruders and can lead to processing difficulties, such as blockages, as well as requiring increased energy for extrusion process. Also the gel particles are potential sources of defects in the final membrane or fiber.
According to the present invention, it has now been found that, by polymerizing ethylene in the presence of a catalyst comprising a Group 4 metal complex of a phenolate ether, it is possible to produce high molecular weight polyethylene, which has a solubility in decalin and other hydrocarbon solvents substantially independent of molecular weight. The resultant polymer is therefore particularly desirable for use in gel processing since higher molecular weight, and hence stronger, fibers and membranes can be produced without a significant increase in processing time.